Why do trainers (not that I have one) always say "breathe while doing your exercises". Well what about situps?

I’m unable to breathe (the inhales and exhales) while doing situps. During the actual crunch I have to hold my breath, or I’m unable to do the exercise.

I just catch quick breaths doing it, in between the crunches.

So how the hell do you breathe during situps? Or is it okay to just hold the breath?